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TAXONOMY OF VASCULAR PLANTS: Part One

05. Phylogenetic considerations

phylogen.htm

by George H. M. Lawrence, Professor of Botany at the Bailey Hortorium, Cornell University, 1951

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Phylogenetic considerations p.092
Significance to taxonomy p.092
Diversity of phyletic concepts p.094
Contributions to phylogenetic knowledge p.097
Phylogeny and the higher categories p.101

 

Footnotes
UKT notes

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05. Phylogenetic considerations

[{p092}]

Phylogeny is the evolutionary history of a taxon, and attempts to account for its origin and development. It is a function of taxonomy, by acceptance of a broad definition of the term. The term phylogeny is the antonym of ontogeny.1 (fn092-01) [UKT: Do not mistake ontogeny for ontology.] A primary objective of phylogenetic studies in botany is the determination of origins and relationships of all taxa of both extinct and present-day plants and the classification of them according to a system that will indicate their genetical or "blood" relationships. A truly phylogenetic classification does not now exist; it is doubtful if it ever will exist, but there is reason to believe that, with the acquisition of many more data and by the synthesis of all available data, a more satisfactory classification than now known may be produced. It should be made clear that this phylogenetic system of the future undoubtedly will be a 3-dimensional (or more) and reticulate in character and that it will be too complex in organization to be of practical use in everyday classification of plants.

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Significance to taxonomy

Phylogeny deals with the evolutionary history of all taxa, from those in the category of division or phylum down to the species and their subdivisions. It is a function of taxonomic research at all levels of classification. A goal of phylogenetic research is the production of a phylogenetic system of classification. In its complete (and probably unattainable) form this phylogenetic system would enable one to determine the ancestor of a plant at any stage of its evolutionary development; it would show

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the genetic and time relationship of any one taxon to another. In this regard it should be noted that there is a distinction between phylogeny and genealogy, for while the former deals with the evolution of the taxon the latter deals with the ancestry of the individual. Likewise, there is a distinction between phylogenetic and taxonomic classification, for (as pointed out by Turrill, 1942, p.685) "taxonomy is based on characters, phylogeny on changes of characters." From this, it develops that there is a distinction between a complex 3-dimensional phylogenetic classification as outlined above and the existing so-called phylogenetic classification. 2 (fn093-02). Present-day classifications may be to the phylogenetic classification of the future what an artificial key is to a synopsis. 3 (fn093-03). Sprague (in Huxley's New systematics, p.441) summed up the situation clearly when he stated that, "in making an artificial classification there is arbitrary selection of characters, no attempt being made to arrive at groups exhibiting a maximum correlation of characters. In attempting to build a natural classification the units ... are arranged in various ways until such maximum correlation is obtained."

It is probable that phylogenetic studies at the level of genus and below have been of greater significance and utility to the taxonomist than have those of the major groups. The reason for this is that the methods of the phylogenist who deals with higher taxa have been limited more or less to those that deal with paleobotanical and morphological research, whereas the taxonomist endeavoring to learn the phylogenetic relationships

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of the components of families, genera, and species has used these methods augmented by cytogenetic and serological research.

It may be asked why, if a present-day classification meets our practical needs, we should strive to piece together a complex phyletic system. The basis of all evolutionary theories is the belief that living organisms may have progressed (though with many digressions) from primitive to more advanced forms. Only a fraction of the total of those forms is known, but the size of the fraction is not known. The living forms about us (species, genera, families, etc.) represent only the tips of branches, end products of evolutionary processes. This being true, the evolutionist -- or phylogenist -- will never be content until the theory of evolution has been proved as a law of nature. This he proposes to accomplish by the discovery and fitting of all evolutionary stages of life into one biological cosmos. A major segment of that cosmos will be a valid phylogenetic classification complete in all details and free from gaps and missing links. A phylogenetic system of classification for plants would provide the answer to questions of their origin, to their modes of evolution, to problems of monophyleticism vs. polyphyleticism, the identity of primitive and advanced characters, etc. It would result in a single stable classification of relationships.

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Diversity of phyletic concepts

There is little unanimity of current opinion on phylogenetic matters. This situation is attested to by the diversity of opinion represented by the presumedly phylogenetic classifications of Bessey, Hutchinson, Wettstein, Pulle, Skottesberg, et al. 4 (fn094-04) The reason for this is the lack of factual data. Botanists do not know enough about the vascular plants of the past (except perhaps of the ferns and conifers) to be able to distinguish with certainty between characters indicative of primitive conditions and those indicative of advanced conditions. The construction of the ultimate phylogenetic classification must be based on established facts regarding the characteristics of ancestors of every taxon level. These ancestors existed in remote geologic time, and because of their relative simplicity their characters are said to be primitive, while those of their present-day descendants are said to be advanced. The primitive characters of contemporary (and presumedly phylogenetic) systems of angiosperm classification are not often based on paleobotanical evidence illustrative of ancestral

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conditions, and for the most part their primitiveness may be a matter of personal opinion or of judgments based on circumstantial evidence. Many of the so-called primitive characters in published lists (including those in this text) are alleged to be primitive because they occur in members of primitive taxa, and the taxa are primitive because they have primitive characters. It is difficult to find, by objective methods, devices to break this cyclic reasoning, and, among the angiosperms at least, there is inadequate paleobotanical evidence to support one view and to reject the other. For example, Eichler, Wettstein, Rendle, and others considered the unisexual apetalous cyclic flower to be primitive, whereas Bessey, Hutchinson, and others treated it as advanced and considered the bisexual polypetalous flower with spiral arrangement of parts to be primitive. The meager paleobotanical evidence of earliest angiosperms (in point of geologic age) is of plant structures identified with taxa representative of both views. Although it is not concrete evidence from the paleobotanical record, but is to a large extent circumstantial and based on assumptions, the evidence that favors the views of Bessey is stronger than that which favors the views of Eichler. 5 (fn095-05)

The lack of paleobotanical evidence hampers and perhaps retards current phylogenetic studies of the angiosperms, but this situation does not mean that the phylogenist must "rest on his oars" and await new paleobotanical data. Rather, he must continue to make the best use possible of all other available evidence. Bailey (1949) expressed this view when he said that "it should be emphasized ... that diversified investigations of surviving angiosperms provide the only available means at present

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of morphologically characterizing this great group of the vascular plants ... " It has been pointed out by phylogenists that there is strong, and seemingly irrefutable, evidence among living plants that certain basic characters or conditions are primitive and that others are derived from them. For example, in leaves of vascular plants as well as in the reproductive parts, the primitive arrangements (in most angiosperms) is in a spiral (supported by the fossil record). Furthermore, in the angiosperms, the anatomy shows that most so-called whorled or cyclic arrangements of floral parts are vertically compressed spirals, and likewise many leaf whorls are not true whorls but are compressed spirals. This sequence is presumed to be evidence that the spiral arrangement is more primitive than is the cyclic. In some apetalous flowers (as in Salicaceae, Juglandaceae, Urticaceae, Gramineae, some Centrospermae, and others) vestigial vascular systems are present that occupy positions anatomically homologous with those in petaloid flowers that lead to perianth parts. This is presented as evidence that apetaly is an advanced condition, derived form ancestors whose flowers had a perianth. Similarly, evidence from living plants supports the view that apocarpous gynoecia (unicarpellate ovaries) are primitive and that syncarpous gynoecia are advanced. Furthermore, these several allegedly primitive characters occur in combination in some families as a positive correlation of high value, a situation that strengthens the view that those families are more primitive than are others represented by similar correlations of lower value or of negative value.

The diversity of phyletic concepts is due also to the lack of synthesis of all available data. In all fairness to Hutchinson, his system cannot be criticized in this regard since he did not accompany his presentation with reasons for the alignment of most of the taxa. However, there is little to indicate that Bessey accepted much evidence other than that provided by gross and comparative morphology in the development of his system (and the tenets on which it was based) from that of Bentham and Hooker. Hallier and Wettstein both drew on the paleobotanical, serological, and anatomical data then available (in addition to morphological considerations), but many data which have since been accumulated are in contradiction to their opinions. Another reason for these diversities is that some phylogenists have given too little attention to the phyletic significance of pollen grain and starch grain morphology, to the physiological bases of serology and allied physicochemical relationships, and to the relationship of genic constitutions to segregation and establishment of major taxa.

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The problems of phylogeny are so complex that only by considered analysis and synthesis of all possible evidence will there be anything approaching harmony of opinion on the subject.

The incompatibilities of some data with other data are responsible in part for the diversity of phyletic views. These resolve themselves into incompatibilities of interpretation of facts, and incompatibilities between bits of circumstantial evidence that too often are treated as facts. Evolutionary patterns laid down in conformance with morphological findings often are at variance with those based on physiological findings; in many instances the fossil record does not support segments of an allegedly phylogenetic classification based on the comparative morphology of extant taxa; in some instances, paleobotanists have alleged that entomophilous flowering plants existed during certain geologic ages that paleontologists have considered to be devoid of insect pollinators; embryological situations presumed by some authorities to be highly advanced occur in flowering plants treated by other botanists as primitive (as in some of the Amentiferae); and primitive characters of wood anatomy sometimes are at variance with allegedly primitive morphological characters of the same taxon. Existence of these conflicting situations should not be construed to imply that a relatively primitive extant flowering plant must possess primitive characters in all its parts, for it is accepted generally that a plant may be primitive in one or more respects and advanced in others. The basis of the conflict often rests on the bias of the phylogenist who may reject (or not even consider) characters contributed by disciplines in which he himself is neither well versed nor a specialist.

The existence of these diversities of phyletic concept should not be discouraging, nor be taken to indicate that all is confusion, but rather should be viewed as part of a healthy situation. The recognition of the causes of the diversities should serve to demonstrate to the phylogenist the need of exhuming more paleobotanical materials, and of carefully integrating with them the findings, based on living material, of research from all fields of biology. The formulation of phylogenetic classifications demands the teamwork and collaboration of botanists of all disciplines and the considered evaluation of data without bias.

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Contributions to phylogenetic knowledge

Paleobotany must be the foundation of phylogeny. The lack of solid and factual foundations for phylogenetic studies is a direct reflection of our knowledge of the paleobotanical history of vascular plants, especially

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of the angiosperms. The situation was expressed well by Turrill (1942, p.508) when he wrote,

The great diversity of opinion in published accounts of plant phylogeny suggests ... that the available data are still too few for the construction of a valid general phylogenetic scheme ... the paucity of relevant paleobotanical data in most groups of plants (partial exceptions are the Pteridophyta and Gymnospermae) is a major cause of uncertainty as to whether or not proposed series are phylogenetic, and, if they be, in which direction they should be read.

One of the requisites of phylogeny is the determination of origins of taxa, not only the origins of those in the highest categories but also those of levels such as class, order, and family. Present-day taxa of these latter categories among the angiosperms originated (presumably) millions of years ago, yet (fide Thomas, 1936) what little is known of them from fossil record is at variance with views of their origin as accepted by most phylogenists. Some phylogenists, lacking necessary data from the paleobotanist, have fabricated missing links to make plausible their otherwise unfounded views (as Arber and Parkin's hypothetical Hemiangiospermae, 1907).

Anatomy is a source of evidence available from both paleobotanical and living material and is of considerable value to the phylogenist. Once the fossil record has clarified the issue of primitive vs. advanced characters, and the issue has been settled as to what type of stem anatomy preceded another, the existing accumulation of anatomical evidence from both vegetative and reproductive parts will be of critical importance. In the interim, much assistance is provided by these facts establishing relationships or probable affinities between taxa in the lower categories, especially below that of the order. Knowledge of the phyletic relationships within the angiosperms, the monocotyledons, and numerous families of dicotyledons, has been advanced by evidence of this type (see papers by Bailey, Bailey and Sinnott, Chalk, Cheadle, Eames, Heimsch, Metcalf and Chalk, Record, and Tippo). Similar and equally significant advancements have accrued from studies of the inflorescence and floral anatomy.

Morphology has dominated phylogenetic research for over a century. Initially it was restricted to gross morphology, and studies of it were followed by a period of intense ontogenetical investigation (an approach to phylogeny defended by Sahni, 1925, and by Lam, 1948, reviewed by deBeer, 1936, and currently de-emphasized by many botanists), and lately superseded by the widespread recognition of the significance of investigations of embryology and floral anatomy. Noteworthy also are the phyletic studies of seeds, especially of embryo and endosperm (Martin, 1947).

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One reason for this dominant position of morphology is that morphological studies permit ready determination and correlation of characters; another is that they can be subjected to comparative analysis to a greater degree than can characters from most other studies. In the phyletic studies of minor categories, it seems probable that much can be learned by the application of quantitative and biometrical methods to an analysis of morphological data (see papers by Anderson, 1936, Anderson and Abbe, 1934, Fassett, 1941, Epling, 1942, et al.). In the determination of phyletic positions within the higher categories, one objective of both anatomical and morphological studies has been evaluation and utilization of characters believed to be the more conservative. 6 (fn099-06) Another objective has been the determination of the direction from which to read a series of transitional morphological situations. In other words, the solution of the basic problem of determining which end of the series is primitive and which is advanced rarely can be based on indisputable evidence.

Cytology and cytotaxonomy are studies which to date have been of most value in the phyletic resolution of taxa below the level of genus. Even then, conditions often have been so uniform that they have contributed little to the understanding of relationships within some genera (Rhododendron, many gymnosperms, cacti, et al.) Cytological data of themselves are inconclusive in determining or strongly indicating phyletic relationships of major categories. An example of this is afforded by one assemblage of data designed to indicate that the Magnoliales may have been derived from wide crosses between different groups of gymnosperms (Anderson, 1934). Since the Magnoliales are phyletically old, these presumed crosses did not occur between present-day gymnosperms, but between ancient ancestors of them. Postulations or hypotheses of what might have happened genetically in previous geological ages generally are more in the realm of philosophy than of science.

Phytogeography, particularly as correlated with the morphology of the earth, undoubtedly will contribute increasingly to the accuracy of phyletic investigations. Too little is known yet about the early land formations of the earth, but with resolution of this hiatus the early migrations of

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plants will be understood better and there must then be harmony between the phyletic arrangement of major categories and the evolution of their distributions. As was true for cytogenetical evidence, so is it true also for phytogeography that evidence contributed by studies of it currently is more important in the phylogentic classification of taxa at and below the level of the family.

Physiology has produced criteria that have been by-passed by most phylogenists, and especially by taxonomists dealing with the minor categories. The contributions of Chester, Reicherf, Molisch, and, to a lesser degree, of Mez attest to the basic value of serological and physicochemical investigations to a better understanding of relationships. The classical serological studies by Molisch and others are in surprising agreement in many respects with several phylogenetic classifications based primarily on morphological evidence. Molisch contributed much to our knowledge of distributions of chemical products among plant genera and families and concluded (to quote Turrill, 1942, p.503) that, "while we are only at the beginning of phytochemical knowledge, the phylogenetic value of phytochemistry is already considerable. Especially can the chemistry of plant substances and their distribution suggest the correctness or otherwise of phylogenetic schemes based on morphological or other criteria." It is probable that further phytochemical studies may aid considerably in confirming or rejecting the transfer of genera from one family to another, or of a family from one order to another. Reichert's work on starches (1919) should be re-examined and evaluated in conjunction with phyletic studies at all levels. Studies of pharmacognosy have provided much information on the presence of such organic materials in plant tissues as alkaloids, glucocides, resins, oleoresins, volatile oils, etc. These are believed in some instances to be indicators of phylogenetic relationships, and further investigations are needed in this direction.

The contributions from the physiological approach are yet in the primitive or initial stages. This in no way lessens their potential value or significance, but rather serves to emphasize the need of revitalized activity by the physiolgist, together with a broader outlook on the basic problems. The collaborating physiologist must admit the possibility that parallel or convergent evolution is present in his field of interest as it has been demonstrated to exist in the disciplines of morphology, cytology, and genetics. The fact that end products of biochemical processes occur in different genera doe not of itself mean that those genera are more closely related to each other than to genera not possessing them. Furthermore

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it is not merely the chemical end product that is of significance, but also the processes by which it was produced. The end product, measured by serological or biochemical tests, may be compared to the processes or materials that produce it as the gross morphology of the plant is to the vegetative and reproductive anatomy of that plant. There is unlimited opportunity for the biochemically trained physiologist who will work in concert with the morphologist, anatomist, and taxonomist on problems of phylogeny of vascular plants at all taxal levels.

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Phylogeny and the higher categories

Pteridophytes
It has been established by numerous researchers that the Pteridophyta are not a phylogenetic taxon (Eames, Andrews, Arnold, Copeland, Wettstein). The available evidence makes it clear that phylogenetically they must be thought of and treated not as one but as 3 or 4 taxa, as: the psilopsids, descended probably from the Psilophytales of the lower Devonian time; the lycopsids (including the Lycopodiaceae, Isoetaceae, and Selaginellaceae) descended perhaps from such ancestral stocks as the lepidodendrids of the early Carboniferous; the sphenopsids (scouring rushes) derived perhaps from calamites and sphenophylls and perhaps the Hyeniales of the lower Devonian; and a fourth taxon, represented by those ferns comprising the Filicales, whose ancestral stocks are traceable back into the Permean (the upper Paleozoic). The paleobotanical evidence in support of these views is more adequate than that known for most other major categories of plants. These major taxa are among the oldest of land plants. Also, they are the oldest of vascular plants, for nearly three-quarters of their evolutionary history had elapsed by the time modern flowering plants were first known and they were old when the cycads and ginkgoes became established. Some of these ancients ferns (as anemias, marattias, angiopteris) are known by living descendants today, and others by descendants of close affinity (as the fossil Osmundites and its contemporary counterpart Osmunda).

The availability of this paleobotanical material has made it possible to reconstruct many of the evolutionary channels of development that lead to present-day vascular cryptogams. It has made it clear that the ferns and their allies are of a polyphyletic origin, that they have had no known common ancestor, and that each of the 4 contemporary major taxa of vascular cryptogams (pteropsids, sphenopsids, lycopsids, and Filicales) may be as different from the others as the club mosses or scouring rushes are from buttercups or orchids. The evolutionary picture of these plants, well known though it may be in a relative sense, is far

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from complete. Many gaps and extensive voids remain to be filled in the almost crude phylogenetic structure now available: parts of that structure have been constructed from weak or even hypothetical evidence, and nothing definite is known of the character or identity of fern antecedents.

Gymnosperms
These are the cycads, ginkgo, taxads, conifers, Gnetales, and some extinct taxa. Counterparts of present-day cycads and the ginkgo date back through the Triassic and into the Permian (Upper Carboniferous), while modern conifers were dominant in the Cenozoic and extended doubtfully into the Upper Cretaceous. Prior to this, there were transition conifers that were climax vegetational types from the Upper Cretaceous back through the Triassic. Conifers ancestral to these lived during the Paleozoic. For much of our knowledge of the Paleozoic conifers we are indebted to the researches by Florin (1938-1945) that have contributed also to a clearer understanding of relationships within modern conifers. These early conifers were trees and shrubs, appeared perhaps more like modern Araucarias than other extant types, with small flattened leaves in spirals, loose strobiloid inflorescences that were intermediate in organization between those of the Cordaites and the present-day pines. The better understanding of these plants and their descendants led Florin to the conclusion (1948) that the taxads are not a part of the Coniferae, but are a separate and equivalent taxon.

Evidence produced during the past few years has supported the views of some phylogenists that the Cordaites, or plants ancestral to them, may have been the principal ancestors of the conifers. The Cordaites are an extinct taxon, of greater antiquity than the conifers, of unknown origin, and are responsible in part the views of some paleobotanists that the origin of the gymnosperms probably extended back into the early Devonian epoch. In any case, the identity of the early ancestors of the conifers becomes speculative, and no fossil plant remains have been discovered that definitely can be said to be ancestral to the cycads or to the ginkgo.

The knowledge of phylogeny within the gymnosperms has advanced appreciably with the increase in knowledge of their paleobotanical background. Within the conifers it has become increasingly clear that the several families are of a more remote relationship to one another than was formerly believed and that the family Pinaceae or older works deserved to be treated taxonomically as an order or suborder, and its tribes raised to the family level. No ancestors are known for the Gnetales, Welwitschiales, or Ephedrales, and none of these orders is closely allied to other gymnosperms. Like the ferns, the gymnosperms are certainly

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of polyphyletic origin, and there is no evidence to suggest a common ancestor for the cycads, ginkgo, taxads, and the conifers. Similarly, there is no evidence to suggest phyletic relationships between the pteridophytes and the gymnosperms; the available evidence suggests that the latter are not derived from the vascular cryptogams.

Angiosperms
The paleobotanical record for the seed plants is so inadequate that phylogenists have had no solid foundation on which to construct a classification of phyletic relationships. Much of the meager paleobotanical evidence is of vegetative parts, and almost none of the reproductive material is derived from the flowering plants more primitive than can be found among modern angiosperms. The lack of this material is responsible to a large extent for the diversity of opinion as to what types of structures are indicative of primitive conditions and what are clearly advanced. Plants of ranalian affinities and those of amentiferous affinities are among the oldest of paleobotanical material. From the evidence avialable, it would seem that the angiosperms "blossomed forth" with a sudden surge in abundance and variety in the late Cretaceous, for fossil material of trees scarcely differentiable from those now growing about us is obtained from strata of that epoch. 7 (fn103-07) Some of this material dates back into the Jurassic, and it is becoming more clear that the angiosperms originated earlier than was indicated by evidence of a quarter century ago.

The origin of the angiosperms is not known. Many theories (some more philosophical than scientific) have been presented to account for their origin; some are supported by circumstantial evidence that is convincing; others are based on more tenuous evidence, but enjoy current acceptance; and a few are so lacking in credibility as to be accepted only by the gullible. There seems not to be a majority of opinion in favor of one view over another, but if the admittedly inadequate and partially circumstantial evidence in support of the pteridosperm is considered satisfactory then that theory is the most difficult to refute. The pteridosperms (seed-bearing plants with fernlike foliage) had their origins in Devonian and were dominant during most of the Carboniferous. They produced pollen from microsporophylls and seeds from their naked ovules. The plants were monocious or dioecious and the probably apetalous solitary megasporangia or ovules were variously disposed (often in panicles). They are the most primitive of known seed plants, but the phyletic gap between them and the earliest true flowering plants of the

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Jurassie is exceeding great. In recounting theories of angiosperm origin Andrews (1947) has said,

... the pteridosperms seem to present the only possible fossils to which we may look as a starting point for the flowering plants. The seed pod of the latter constitutes their most distinctive feature, and the cupule which encloses the seed (or seeds) in the pteridosperms is the most likely precursor of the flowering-plant seed pod. It has not been proved that this is actually the case, but since it is the most plausible and convincing evidence that we have to go on, then it is justifiable to use it at least as a working hypothesis until more evidence is accumulated either to support it or to disprove it.

A similar view was taken by Arnold (1947), who stressed also that the angiosperms must have had "a longer and more extensive pre-Cretaceous history than so far has been revealed by the fossil record and that the scarcity of fossils is due to their predominatingly upland habits where the remains were not readily buried and preserved." Darrah (1939) conceded the pteridosperms to be probable ancestors of the cycads, but held that "relationship of the seed-ferns to groups other than the true ferns and cycads, is entirely debatable." He was of the opinion that the angiosperms arose "without shadow of doubt from some gymnospermous stock."

It is the opinion of other paleobotanists and phylogenists that the angiosperms have been derived from the gymnosperms or from stocks ancestral to them. In 1907 Arber and Parking endeavored to established this theory by the fabrication of a hypothetical connecting link that they called the Hemiangiospermae, in which the reproductive structure was constructed according to the plan of the cycadeoid flower; a structure assigned a perianth of many distinct parts, open foliaceous carpels with marginal megasporangtia, and an androecium of many stamens, all aranged in spirals. There is no evidence in the fossil record that such a structure ever existed, and without it their theory of angiosperm origin from the gymnosperms via cycadaceous ancestors has no substance. Despite this absence of fact, the theory was accepted by Bessey, by Hutchinson, and others. Hagerup postulated a diphyletic origin for the angiosperms with one line extending from the Filicales through the cycads to the Ranales (the Poycarpicae) and the second from the lycopods through the Cordaites, conifers, and Gnetales to such angiosperm taxa as the Centrospermae and Personatae. 8 (fn104-08) These views were rejected in part by Chadefaud (1946), who pointed out the incompatibility of their basic tenets with morphological conditions known to exist in the ovules of pteridosperms, cycads, and some modern flowering plants. The significance

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of the Gnetales to the solution of problems of angiosperm origins has been emphasized by Markgraf (1930), who held the angiosperms to be a taxon derived from gymnosperms, and one that branched off from ancestors of modern gymnosperms so long ago as to be of no particular relationship to present-day conifers.

 

 

 

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Footnotes

fn092-01 Ontogeny differs from phylogeny in that it accounts for the life history of the individual plant from its development from the zygote to the production of its own gametes. Ontogenetical studies deal also with the development of structure from the stage of primordial initiation to full maturity; phylogenetical studies of individual structures within a plant deal with the comparison of their evolutionary changes through successive generations from the time of origin to the present.
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fn093-02 Distinctions between the 3 classification types (artificial, natural, and phylogenetic) have been given (03. History of Classification, p013; Period 4. Systems based on phylogeny). In using these terms, one should note that the term natural classification has been used by many contemporary botanists as synonymous with phylogenetic classification. In this text, the term natural is restricted to the classifications based on form relationships. Turrill (1942) emphasized the confusion and ambiguity that has resulted from the indiscriminate use of the term "natural" when applied to both pre-Darwinian and post-Darwinian classifications, and has advocated that it be abolished with regard to classification, and the term "general" be substituted for it. Turrill pointed out that none of the present-day so-called phylogenetic classifications is truly phylogenetic, but is only presumed to be so, or is phylogenetic only in so far as available evidence allows. For this reason he extended the application of the term "general" to cover also all modern post-Darwinian systems of classification, and reserved the term "phylogenetic classification" for a system to be developed at a future time when all the facts of evolution (now lacking) have been discovered. The validity of Turrill's views is patent, but their application is deferred for the present because (1) the term "natural" is deeply entrenched in biological literature and thinking, its original usage from before the time of Linnaeus through the nineteenth century is clear and consistent, and since the ambiguities are of relatively recent origin it is to be hoped that they may yet be eliminated if contemporary biologists will be more precise and restrictive in their use of the term; and (2) the term "phylogenetic" implies a classification based on evolutionary sequences and genetic relationships, and since this has been the underlying principle of the more recent modern classifications, then they are phylogenetic in principle as contrasted with the earlier natural classifications.
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fn093-03 For an explanation of the distinction between an artificial key and a synopsis, see Chapter 10, p.225.
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fn094-04 The omission of Engler's system from this listing is deliberate, since Engler did not consider his system to be a phylogenetic classification in the broad sense of the concept (for explanation, see Chapter 06)
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fn095-05 Studies by Sporne (1948, 1949) have emphasized the utility of biometrical correlations between floral and vegetative characters in assessing the relative advancement dicotyledonous families. In arriving at his correlations, he defined a primitive character as "one which, possessed by some present-day families, was also possessed by their ancestors", and a primitive family as "a present-day family which has retained a relatively large number of primitive characters and which has diverged very little from the ancestral". By statistical analysis dealing with presence or absence of 12 allegedly primitive characters throughout 259 dicot families, Sporne arrived at an "advancement index (%)'' for each family. The characters selected to be most primitive were:
   Trees or shrubs      Petals free
   Leaves glandular    Stamens pleiomerous
   Leaves alternate     Carpels pleiomerous
   Leaves stipulate      Seeds arillate
   Flowers unisexual   Seeds with two integuments
   Flowers actinomorphic  Seeds with integument bundles
 As a result, the most primitive dicot families were concluded to be the Flacourtiaceae, Anonaceae, Magnoliaceae, Myristicaceae, and Euphorbiaceae, while the most advanced included the Labiatae, Valerianacea, Dipsacaceae, Phrymaceae (the Compositae standing about two-thirds the distance up the scale. The amentiferous families were scattered but averaged about midway along the scale of advancement with the Fagaceae the most primitive and Garryaceae the most advanced.
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fn099-06 By conservatism of characters is meant their persistence within the plant over an extensive period of evolutionary development. For example, anatomists generally hold to the view that characters of stem anatomy are very conservative, since they vary little or not at all among species of a given genus, rarely between genera of a given family (when they do that genus is suspected by the anatomist of being taxonomically out of place), and that members of many families possess common or similar characters of stem anatomy. The morphologists also generally contend, for example, that the vascular anatomy leading to gynoecia and androecia are modified more slowly than the gross gynoecial and androecial structures, and therefore by their conservatism the vascular conditions serve to indicate probable ancestral situations.
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fn103-07 For an analysis of reasons accounting for this gap in the paleobotanical record, see Just (1948, pp.97-98)
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fn104-08 For a summary and analysis of these and other contemporary views, see Just (1948)
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UKT notes:

(in alphabetical order)

genealogy

genealogy n. pl. genealogies 1. A record or table of the descent of a person, family, or group from an ancestor or ancestors; a family tree. 2. Direct descent from an ancestor; lineage or pedigree. 3. The study or investigation of ancestry and family histories. [Middle English genealogie from Old French from Late Latin geneālogia from Greek genea family; See gen …- in Indo-European Roots. -logia -logy ] -- AHTD

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ontogeny

ontogeny n. pl. on·tog·e·nies 1. The origin and development of an individual organism from embryo to adult. Also Called ontogenesis. -- AHTD

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ontology

ontology n. 1. The branch of metaphysics that deals with the nature of being. -- AHTD

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phylogeny

phylogeny n. pl. phylogenies 1. The evolutionary development and history of a species or higher taxonomic grouping of organisms. Also Called phylogenesis . 2. The evolutionary development of an organ or other part of an organism: the phylogeny of the amphibian intestinal tract. 3. The historical development of a tribe or racial group. [Greek phulon race, class; See bheu …- in Indo-European Roots. -geny ] -- AHTD

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End of TIL file